Citation impact indicators nowadays play an important role in research evaluation, and consequently these indicators have received a lot of attention in the bibliometric and scientometric literature. This paper provides an in-depth review of the literature on citation impact indicators. First, an overview is given of the literature on bibliographic databases that can be used to calculate citation impact indicators (Web of Science, Scopus, and Google Scholar). Next, selected topics in the literature on citation impact indicators are reviewed in detail. The first topic is the selection of publications and citations to be included in the calculation of citation impact indicators. The second topic is the normalization of citation impact indicators, in particular normalization for field differences. Counting methods for dealing with co-authored publications are the third topic, and citation impact indicators for journals are the last topic. The paper concludes by offering some recommendations for future research. Given an initial set of relevant publications on a certain topic, I used CitNetExplorer to identify additional publications that could potentially be of relevance. This was done based on citation relations between publications. For instance, all publications cited by or citing to publications already classified as relevant were identified. Or alternatively, publications with at least a certain minimum number of citation relations (e.g., three or four citation relations) with publications already classified as relevant were identified. For each publication identified by CitNetExplorer, I then manually determined (e.g., based on the title and abstract of the publication) whether the publication is indeed of relevance to the topic of interest 5 or not. In this way, the set of relevant publications was extended. The above steps could then be repeated in order to identify additional relevant publications. A number of iterations were usually performed until all relevant publications on a certain topic seemed to have been found. Two further comments should be made on the way in which the publications included in this review were selected. First, it should be emphasized that the primary aim of this review is to provide an overview of the current state of the art in the literature on citation impact indicators. The focus of the review therefore is mainly on the more recent literature on citation impact indicators. No special attention is paid to the historical development of the literature. Second, it should be mentioned that this review does not provide an exhaustive overview of the literature on citation impact indicators. Given the size of the literature, providing an exhaustive overview in which all relevant publications are included is hardly possible. As already pointed out inSection 1, this review focuses on selected topics studied in the literature on citation impact indicators. However, even the literature on these selected topics cannot be covered in a fully comprehensive way. This review therefore includes ...